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Seacow's got a great reputation for cab IRs as far as I've heard. Just enter your email, and Audio Assault will send you a download link.
Seacow Teaser Pack
This is the new free pack which includes 13 IRs from upcoming cab packs.
The new Cab Packs will have 4 different speakers per cab, 15+ mics including ribbons, dynamics and condensers.
impulse response (IR) is effectively something that's applied to a guitar signal to simulate a guitar cabinet. Some refer to IRs simply as cab sims, though purists will complain as they always do.
In short, if you use an amp sim without any sort of cab sim, your tone will sound really fizzy with too much high frequency noise. Real speakers cut off most sounds above 8khz, which is basically means it takes the sound down, and makes it less fizzy and more pleasant to listen to.
Audio Assault make some good stuff — I'm particularly fond of their 'Sigma' amp sim. Not the best stuff in the world, but their products are usually a fair bit cheaper than the competition, so there's that.